1. Field of the Invention
The present invention relates generally to a jet pump, and more specifically to a process for optimally sizing the length of a constant area throat of a jet pump.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Jet (or ejector) type pumps are commonly used in many applications, including water pumps, industrial chemical pumps, sewage pumps, etc. FIG. 1 illustrates a common jet pump configuration. The benefit of a jet pump is the increased reliability through lack of rotating machinery. This also makes them ideal for applications where fluid with solid particles is being pumped.
The performance of the jet pump comes from the way the fluid pressure is increased—a jet of higher momentum fluid is used to “drive” the pump. That is, the higher momentum jet mixes with the lower momentum fluid to be pumped in the constant area throat, imparting its work on the fluid in that manner. Then the total pressure of the mixed flow is recovered into an overall static pressure rise through the diffuser.
In order to recover the maximum amount of pressure from the diffuser, a high divergence angle is necessary. For high divergence angle diffusers to work optimally, the mixing of the driving (primary/high pressure) and driven (secondary/low pressure) flows has to be complete by the exit of the constant area throat in order to avoid separation in the diffuser.
For low area ratio (primary nozzle area/throat area) jet pumps, there is an efficiency limit based on how much surface area the high energy jet has to mix with the low head fluid. The potential core is the region of essentially uniform velocity on a jet, which is circumferentially surrounded by the shear mixing layer. See FIG. 2 for an illustration of jet potential core flow. For conventional jet pumps, there are several rule-of-thumb guidelines for how long to make the constant area throat, with lengths anywhere from 3-9 times the diameter of throat being recommended. However, the longer the throat length, the more wall friction losses are encountered. Therefore, a designer wants to make the throat just long enough to complete mixing but no longer due to the wall friction losses.